Anionic polymerization has been widely applied in the field of polymer synthesis, due to its advantages, such as fast initiation and propagation, almost no termination, controllable molecular weight, narrow molecular weight distribution, and definite molecular structure. Under a proper condition, the chain termination or transfer reactions would not be happened in the anionic polymerization, the living chain could preserve polymerization activity until the monomers are exhausted. Functionalized telechelic polymers or block copolymers with deferent structures could be obtained by adding proper endcapper or a second kind of monomers. However, because of the extremely fast polymerization rate, great amount of heat instantaneously produced during anionic polymerization would lead the reaction hardly to control in its industrialization. Reducing polymerization temperature could result extremely high viscosity of intermediate products, especially for high concentration solution or bulk polymerization, which could cause the difficulties of stirring and instability of products. Furthermore, the molecular weight of the final products would be changed when the dosage of initiator is reduced. Therefore, all the industrialized anionic polymerization utilizes solution polymerization. Furthermore, adding a massive amount of solvent to dilute the solution and decrease the viscosity will lead to the solvent reprocessing and environment problems. In addition, for the anionic polymerization of certain monomers (especially alkadiene and monomers containing ester group), the excessive rate or higher temperature easily lead to a great amount of side reactions, even no target product can be obtained, which seriously hinders the industrialization process of anionic polymerization. To date, only a limited kinds of monomers could be applied in industrialized ionic polymerization.
Therefore, research on the additives to control and adjust anionic polymerization has not been interrupted. Certain additives and related theory have also been put forward. Shengkang Ying (Shengkang Ying, Shaohua Guo et al. Ionic Polymerizations. Beijing: Chemical Industry Press, 1988), Guantai Jin (Lianbao Xue, Guantai Jin. Theory and Application of Anionic Polymerization. Beijing: China Friendship Publishing Company, 1990) et al reported that using polar modifiers (THF, diethylene glycol dimethyl ether, N,N,N,N-tetramethylethylenediamine) could disassociate the initiator aggregations of anionic polymerization, and all the initiators were initiated simultaneously. The use of such polar modifiers not only accelerated the polymerization rate, made the polymerization could be carried out at lower temperature, but also narrowed the molecular weight distribution to near 1. The methods made a great contribution to the theory and applications of anionic polymerization, but could only accelerate polymerization rate, decrease the reaction temperature and increase the amount of random products. However, there is no effective reverse controlling method, i.e., which can reduce the reaction rate, decrease side reaction and increase reaction temperature. So the mentioned methods could not solve the problems of gels and other side reactions in the process of bulk and high concentration solution polymerization of styrene butadiene rubber, and the problem that many reactions have only to be carried out at dozens of degree below zero, which made the methods lack practice industrial value. Intense application demand promotes the research on this field.
At the beginning, the cocatalysts added in coordination polymerization were directly used into anionic polymerization systems. For example, early in the 1960s Welch F. J. reported the influence of adding Lewis acids or bases into the n-BuLi catalyst system on the polymerization rate of styrene anionic polymerization [Polymerization of styrene by butyllithium. II. Effect of Lewis acid and bases. Journal of the American Chemical Society (1960), 82, 6000-5]. The result shown that a small quantity of Lewis bases (such as ethers, etc) could accelerate the anionic polymerization rate of styrene, while Lewis acids (such as alkyl aluminums) would decrease the polymerization rate. The polymerization would be ceased if adding more than the stoichiometric amount of Lewis acids.
Hsieh and Wang studied the coordinated compounds formed from dibutylmagnesium with alkyllithium initiator and/or living polymer chains with or without THF presence. They found that dibutylmagnesium could decrease the polymerization rate of styrene and butadiene, but had no influence on its stereochemistry (Macromolecules, 19 (1966), 299-304).
Patent CN 1646580A reported an anionic polymerization method using composed initiator, which contained at least one alkali metal hydride selected from LiH, NaH and KH, and at least one organic aluminium compound. Among them, the alkali metal hydride was used as anionic polymerization initiator, and the organic aluminium compound could improve the solubility of the alkali metal hydride in solvent. The organic aluminium compounds could not only improve the activity of alkali metal hydride through coordination, but also reduce the polymerization rate of monomers.
Patent CN 1291205A reported a technique to delay anionic polymerization. Monomers were polymerized in the presence of at least one organic alkali metal compound, at least one organic magnesium compound and at least one organic aluminium compound. A composed initiator which could adjust the polymerization rate in a wide temperature and concentration range was also provided. The composed initiator contained organic magnesium compounds and organic aluminium compounds.
Patent CN 1291207A reported a technique of preparing block copolymer via delayed anionic polymerization, i.e. a method of block copolymer synthesized from vinyl aromatic monomers and diolefine. The polymerization was carried out in the presence of at least one organic alkali metal compound or alkali metal alkoxide, as well as at least one organic magnesium, aluminium or zinc compound. The synthesized block copolymers contained S-B-S, S-S/B-S, S-B-S/B-S and other block structures.
All the above patents and literatures described the method and examples delayed anionic polymerization reaction in detail, which were almost according to the same idea, i.e. using a composite to coordinate with the initiator to form a certain complex structure. For example, Alain Deffieux suggested that the hydride and organic aluminium compound can coordinate with initiator as the following formula [Polymer 46 (2005), 6836-6843]:PSLi+i-Bu3Al≈i-Bu3Al:PSLi(1:1) Strong coordinationi-Bu3Al:PSLi+PSLi≈i-Bu3Al:(PSLi)2(1:2) Weak coordination
In the anionic polymerization system of (methyl)acrylic esters and other unsaturated monomers containing carbonyl group, the presence of carbonyl group will lead to a series of side reactions. Generally, a pre-endcapper such as 1,1-diphenylethlene (DPE) was used to increase the steric hindrance of initiators, reduce the polymerization temperature, and prevent carbonyl to take part in the reaction as much as possible. The pre-endcapper DPE can only efficiently avoid the side reactions of the carbonyl in the first monomer, but it can not efficiently avoid the carbonyl in the second monomer to take part in the reaction, because the influence of the steric hindrance becomes less or even disappears after the first monomer is added to the carbon anion of the initiator. At low temperature, the reaction activity of carbonyl group is very low, DPE can show its inhibition effection. However, with the increase of the reaction temperature, the reactivity of carbonyl becomes higher, adding DPE will have no inhibition effect.
Most of the above methods, especially the proposed theories are conjectures, and lack direct and effective experimental evidences. Therefore, they can not explain the relationships between these mechanisms and polymerization temperature accurately. Furthermore, this coordination compound system needs two or more substances and the feeding process is relatively complicated. Organic magnesium and aluminium compounds are inflammable and explosive, and can easily react with the oxygen and moisture in the air. Therefore, their transportation and processing are difficult. These compounds are very insecure in manufacturing production. Meanwhile, the control on the amount of additives requires high precision; otherwise the influence to products will be distinctive. In addition, these coordination compounds have relatively strong nucleophilicity, so that they will directly influence on the fictionalization of follow-up products. For example, in the synthesis process of hydroxyl-terminated polybutadiene, epoxy ethane is directly added in the final stage of butadiene anionic polymerization, then terminated by methanol, the target product can be obtained; If there are organic magnesium or aluminium compounds remained, ring-opening reaction with the added epoxy ethane will happen, resulting in byproducts and lose of reagents. Therefore, these coordination compounds have a big problem not only in theory but also in application.